412 Precondition Failed

The 412 Precondition Failed status code is an HTTP response status that indicates a client-specified precondition was not met by the server. When a client sends an HTTP request with certain preconditions, the server must evaluate these preconditions to determine if they are valid. If they are not, the server returns the 412 Precondition Failed status code to inform the client of the failure.

Purpose of 412 Precondition Failed

The primary purpose of the 412 Precondition Failed status code is to facilitate conditional requests. Conditional requests are HTTP requests that include one or more preconditions, which must be met by the server for the request to be processed. These preconditions are specified using HTTP headers, such as If-Match, If-None-Match, If-Modified-Since, and If-Unmodified-Since. By using these headers, clients can ensure that they are working with the latest version of a resource or avoid unnecessary data transfers.

Usage Scenarios

If-Match

The If-Match header is used to ensure that the client is working with the latest version of a resource. This is particularly useful when updating a resource to prevent conflicts caused by concurrent updates.

Request:

PUT /resource/123 HTTP/1.1
Host: example.com
Content-Type: application/json
If-Match: "etag_value"

In this example, the client is attempting to update the resource with the ID 123. The If-Match header is included with the etag_value to ensure that the update only occurs if the server’s current version of the resource matches the etag_value.

Response:

HTTP/1.1 412 Precondition Failed
Content-Type: application/json

If the server’s current version of the resource does not match the etag_value, the server returns a 412 Precondition Failed status code, indicating that the update cannot be performed.

If-None-Match

The If-None-Match header is used to avoid unnecessary data transfers when retrieving a resource. This is useful to minimize bandwidth usage, especially when caching resources.

Request:

GET /resource/123 HTTP/1.1
Host: example.com
If-None-Match: "etag_value"

In this example, the client is attempting to retrieve the resource with the ID 123. The If-None-Match header is included with the etag_value to ensure that the resource is only retrieved if the server’s current version of the resource does not match the etag_value.

Response:

HTTP/1.1 412 Precondition Failed
Content-Type: application/json

If the server’s current version of the resource matches the etag_value, the server returns a 412 Precondition Failed status code, indicating that the resource does not need to be retrieved.

Summary

The 412 Precondition Failed status code is an essential part of conditional requests in the HTTP protocol. It allows clients to specify preconditions using headers like If-Match and If-None-Match, which the server must evaluate before processing the request. If the preconditions are not met, the server returns a 412 Precondition Failed status code to inform the client of the failure.